1. Field of the Invention
The present invention relates to a laser sputtering apparatus and method for producing thin films and, more particularly, to a laser sputtering apparatus and method using a laser frequency selected to excite the target atoms into a reactive state.
2. Statement of the Problem
Thin films of direct bandgap II-VI compounds are important photovoltaic materials useful in optics, photonics, and microelectronics. A variety of thin film deposition methods for II-VI compounds are described in "Thin-Film ZnO--Properties and Applications" by Frans C. M. Van de Pol published in Ceramic Bulletin Vol. 69 No. 12 (1990). Included are methods of chemical vapor deposition, evaporation of Zn with an oxygen beam, high-frequency plasma evaporation, molecular beam epitaxy, and conventional radio frequency (RF) reactive sputtering of Zn in an oxygen ambient.
Sputtering is a well-known technique for producing thin films. Conventional sputtering systems use RF energy to create a plasma inside a low-pressure vacuum chamber. The plasma comprises ions that are directed at a surface of a sputter target. The plasma gives the ions a great deal of kinetic energy so that as they impact the target surface, they remove clusters of atoms or molecules from a surface of a sputter target. The atoms and molecules that are removed then travel through the low-pressure vacuum chamber to deposit onto a surface of a substrate.
In some sputtering systems, the sputtered atoms are caused to chemically react with reagent gases in the vacuum chamber as they leave the surface of the target. These techniques are called "reactive sputtering" and are useful for providing some types of molecular thin films. Because chemical reactions between the group II metals and the group VI reagent compounds are generally endothermic, reactive deposition methods of these materials require the input of energy from some external source. The external energy can be from substrate heating or RF glow discharge through the reagent gases.
Low-temperature deposition is desirable where the substrate upon which the film is deposited contains material that is sensitive to high temperature. One advantage of sputtering is that the thin film deposition occurs at moderate temperature. However, RF plasma sputtering still requires elevated temperatures of several hundred degrees Celsius, and may require that the substrate itself be heated. Reactive ion sputtering often requires even higher temperatures to give the ions sufficient energy to induce a chemical reaction. A method for low- or near-room-temperature thin film deposition is needed.
RF energy can damage delicate microelectronic structures in a substrate. Also, it is often difficult to control the stoichiometry of reactively sputtered films because the RF plasma contains ions with a wide variety of energies which react with the sputtered atoms to create a correspondingly wide variety of compounds. This is a particular problem when reactive sputtering is performed at low temperature. What is needed is a thin film deposition apparatus that provides highly reproducible and uniform thin films without requiring RF energy to cause a chemical reaction.
U.S. Pat. No. 5,017,277 issued to Yoshida et al. on May 21, 1991, describes a non resonant laser sputtering apparatus. This apparatus uses a laser to sputter material from a target located inside a vacuum chamber to a substrate mounted inside the vacuum chamber. The apparatus optionally includes an RF plasma mechanism to allow reactive sputtering and to prevent decomposition of the sputtered molecules. In all cases, the target comprises substantially the same material as the sputtered product.
"Laser-Induced Sputtering of ZnO, TiO.sub.2, CdSe, and GaP Near Threshold Laser Fluence" by Takeyoshi Nakayama published in Surface Science Vol. 133 (1983) describes a method of laser sputtering zinc oxide. This paper reports results of the mechanics of non resonant laser sputtering at various laser intensities. In all of the experiments, the sputter product was either substantially similar to the composition of the sputter target or included ions decomposed from the sputter target. No reactive sputtering is reported.
"Laser Induced Deposition of Zinc Oxide" by R. Solanki and G. J. Collins published in Applied Physics Letters Volume 42(8) (1983) describes a chemical vapor deposition system using non-resonant laser energy to decompose dimethylzinc and nitrogen dioxide to form zinc oxide. This method did not involve sputtering and was instead concerned with decomposition of a zinc compound as opposed to reactively sputtering zinc.
"Highly Oriented ZnO Films Grown by Laser Evaporation" by H. Sankur and J. T. Cheung published in Journal of Vacuum Science Technology vol. 1, no. 4 (1983), describes a method of using a non resonant laser to heat a target to evaporate material from the target onto a substrate. The evaporated films were substantially similar in composition to the target. No reactive evaporation or sputtering is reported.
A need exists for an apparatus and method for forming uniform thin films of II-VI compounds at low temperature without RF fields. A further need exists for a method of laser sputtering that induces a chemical reaction between the sputtered atoms and reagent gases without requiring additional external energy.
3. Solution of the Problem
The present invention provides a solution to the above problem by providing an apparatus and method for laser sputtering atoms from a target comprising a group II metal. A laser frequency is chosen to be near-resonant with an electronic transition in the group II metal from a ground state to a metastable state. As the sputtered atoms leave the target surface, the near-resonant laser energy excites the sputtered atoms. The excited atoms are sputtered into the vacuum chamber, where they react with the group VI reagent gases to form a II-VI compound that deposits as a highly uniform polycrystalline thin film on a substrate located in the vacuum chamber.